Video memory mapping

I'm not sure if this is a question for a videocard group or a
mainboard group or in fact where it belongs. The programming videocard
group is full of spam so I'm posting this to matrox and nvidia because
those two are representative.

I have a series of legacy programs dating from the eighties and early
nineties which were developed using a compiler and a screen generator
that insert into the generated executables a "default memory-mapped
video routine" about which they say "This is seldom a problem..."
Maybe it wasn't back in the days of the 386 when SVGA was state of the
art but it sure is today.

Up until about three weeks ago, these executables with their
"memory-mapped video" worked fine on a series of machines starting
with a 286 and ending with a Celeron on a Tyan board using a Matrox
G200 video adapter and running NT4. Then, as a result of desire to
upgrade and acquire use of the USB, I purchased a Biostar with an S3
Pro Savage 8 integrated running XP Home. Disaster! The legacy programs
stopped working or gave bizarre video results such as a screen of "!"
in a stepped fashion.

At first I blamed the OS as I'd already tried the programs under Win2K
and they worked fine. But then using DOS 6.2 from a diskette on the
Biostar machine I had the same problem. It wasn't the OS! Similarly
the Tyan machine has been fitted with a 3D Force MX 400 64 and it too
produces the bizarre video results or freezes the machine. So I don't
think it's the motherboard.

It's the videocard: Matrox good: S3 Force and S3 Pro Savage bad!
However the Matrox card is slightly old and puny and I'd hate to be
stuck with the same video adapter for the rest of my life. OTOH, I can
hardly buy a series of cards and go through the hassle of testing each
to see which works.

Obviously there's a conflict with the video and my programs fighting
for the same memory addresses.

Question: Where is "default memory-mapped video"? This must be an
industry term and describe a section of real memory, x"F000" through
x"FFFF" for example?

Qusetion: Who decides where the video board will be in memory? I.e.,
what address does it map to and how can I find this out for each
board?

Obviously I'm considering throwing out the S3 Pro Savage and buying a
new board which will not conflict with the "default memory-mapped
video". Does anyone have a better suggestion?

(Note: The compiler mfg and screen generator mfg no longer support
their eighties versions and upgrading to a current compiler and SG
would cost in excess of $3000-- yeah, three big ones-- I could buy a
separate machine cheaper than that! Upgrading to another language/SG
would be around two man years work, something that I'll eventually do
but I need time. These are not games BTW and there's no current
substitute. If there were it'd be a tiny niche market and would
probably cost $3000 too.)